In simple terms atheism means not believing in God - any god. The prefix "a" tells us that the term refers to no god, in the same way as amoral refers to someone with no morals.

Agnosticism on the other hand refers to someone who does not know - who cannot make up their mind. So the difference is very important.

Some people who do not believe in any god still do not like the term atheist as it implies there is a god not to believe in. Strange, heh?

Another problem with the atheist scenario is that there are many definitions of god, an external entity, an internal entity, love, goodness, etc., etc. When discussing the subject one can never be sure how the other person perceives god, and therefore it can be confusing when stating that one does not believe in it.

An agnostic may be someone brought up with a religious belief who now finds it difficult or impossible to hold to it, but at the same time feeling there is something out there. With this belief, they cannot be considered atheist.

Before discussing or considering this distinction it is important to be clear what you mean by god - otherwise both the terms atheism and agnosticism can become at the same time rather meaningless and certainly easily confusing.